1. Field of the Invention
The present invention relates to charge amplifier circuits having compensating characteristics. It is especially suited to circuits used to amplify the signals of piezoelectric hydrophones, especially signals of hydrophones using a piezoelectric polymer material.
2. Description of the Prior Art
There are many known ways of making acoustic antennas with piezeolectric sensors. The material of these antennas is a piezeoelectric polymer or co-polymer such as PVF 2.
FIG. 1 shows a sectional view of the structure generally used for a sensor of this type.
This sensor has two polymer layers 10 and 11, separated by a conducting electrode layer 12. These two layers are biased in the direction of their thickness, and in opposite directions. Their outer side is covered with conducting electrodes 13 and 14. The electrical output signal Ve is taken between the central electrode and these two external electrodes which are connected together.
Each of these sensors, called surface sensors, may have the shape of a rectangle of considerable size, with an area that may exceed 100 cm2.
There are known ways to connect these sensors in parallel to make a large-sized, flat acoustic antenna.
Owing to their flexibility, these sensors can also be given other shapes. For example, they may be given a cylindrical shape, by being placed inside or outside an insulating cylinder.
A common way of amplifying the output signal Ve of the sensor is to use a charge amplifier, the diagram of which is shown in FIG. 2.
The sensor 20 is connected by one of its electrodes to the input of a differential amplifier 21 with very high gain and very high input impedance. This type of amplifer, called an operational amplifier, is quite common. The + input of this amplifier is connected to the ground, like the other electrode of the sensor.
The output of the amplifier 21 is connected to its- input by means of a feed-back capacitor 22 with a capacitance C.sub.r.
From the electrical point of view, the sensor 20 is a capacitor with a capacitance C.sub.h. The output signal V.sub.s is therefore given by: ##EQU1##
Owing to the dimensions of the sensor, the capacitance C.sub.h is relatively great and, by choosing a value C.sub.r which is very small compared with C.sub.h, the amplification of the assembled unit is substantially equal to: ##EQU2##
The sensor 20 not only reacts to the sound signals that it receives by delivering a variable signal V.sub.e, bu is also sensitive to several external parameters, especially hydrostatic pressure and temperatue, which cause the capacitance C.sub.h to vary. This causes considerable variation, as regards both amplitude and phase, in the transfer function V.sub.s /V.sub.e. The result of this is a high degree of measuring uncertainty. This disadvantage is especially great for acoustic antenna mounted on vehicles, such as submarines or towed sonar devices, with varying depths of immersion.